On Tue, Dec 08, 2015 at 06:52:50PM +0300, Andrey Ryabinin wrote:
> tcache_invalidate_node_pages() temporarly drops/takes back node->tree_lock.
> Once lock was dropped, another thread might remove and free the next slot.
> Don't drop the looks.
>
> https://jira.sw.ru/browse/PSBM-42104
>
> Signed-off-by: Andrey Ryabinin <aryabi...@virtuozzo.com>
> ---
> mm/tcache.c | 13 +++----------
> 1 file changed, 3 insertions(+), 10 deletions(-)
>
> diff --git a/mm/tcache.c b/mm/tcache.c
> index b8757cf..9bb88b4 100644
> --- a/mm/tcache.c
> +++ b/mm/tcache.c
> @@ -121,8 +121,9 @@ static struct tcache_lru *tcache_lru_node;
> /*
> * Locking rules:
> *
> - * - tcache_node_tree->lock nests inside tcache_node->tree_lock
> - * - tcache_lru->lock is independent
> + * tcache_node->tree_lock
> + * tcache_node_tree->lock
> + * tcache_lru->lock
> */
>
> /* Enable/disable tcache backend (set at boot time) */
> @@ -677,16 +678,8 @@ tcache_invalidate_node_pages(struct tcache_node *node)
> radix_tree_for_each_slot(slot, &node->page_tree, &iter, 0) {
> page = radix_tree_deref_slot_protected(slot, &node->tree_lock);
> BUG_ON(!__tcache_page_tree_delete(node, page->index, page));
> - spin_unlock(&node->tree_lock);
> -
> tcache_lru_del(page);
> put_page(page);
> -
> - local_irq_enable();
> - cond_resched();
> - local_irq_disable();
> -
> - spin_lock(&node->tree_lock);
This might be OK as a temporary solution, but I think we still should
drop the lock periodically, because this iteration can take long (think
of a 10G file cached in tcache) so performing it w/o rescheduling and,
what is worse, with irqs disabled is a no-go IMO.
I would propose to make use of the ability of radix_tree_for_each_slot
to continue iteration from the specified index. Something like this
would do the trick I suppose (NOT TESTED):
diff --git a/mm/tcache.c b/mm/tcache.c
index b8757cf354a9..6a4c45970293 100644
--- a/mm/tcache.c
+++ b/mm/tcache.c
@@ -662,6 +662,10 @@ tcache_invalidate_node_pages(struct tcache_node *node)
struct radix_tree_iter iter;
struct page *page;
void **slot;
+ pgoff_t index = 0;
+ int batch;
+
+#define TCACHE_INVALIDATE_BATCH 128
spin_lock_irq(&node->tree_lock);
@@ -674,19 +678,26 @@ tcache_invalidate_node_pages(struct tcache_node *node)
* Now truncate all pages. Be careful, because pages can still be
* deleted from this node by the shrinker or by concurrent lookups.
*/
- radix_tree_for_each_slot(slot, &node->page_tree, &iter, 0) {
+restart:
+ batch = 0;
+ radix_tree_for_each_slot(slot, &node->page_tree, &iter, index) {
page = radix_tree_deref_slot_protected(slot, &node->tree_lock);
- BUG_ON(!__tcache_page_tree_delete(node, page->index, page));
- spin_unlock(&node->tree_lock);
-
+ index = page->index;
+ BUG_ON(!__tcache_page_tree_delete(node, index, page));
tcache_lru_del(page);
put_page(page);
- local_irq_enable();
- cond_resched();
- local_irq_disable();
-
- spin_lock(&node->tree_lock);
+ if (++batch > TCACHE_INVALIDATE_BATCH) {
+ spin_unlock_irq(&node->tree_lock);
+ cond_resched();
+ spin_lock_irq(&node->tree_lock);
+ /*
+ * Restart iteration over the radix tree, because the
+ * current node could have been freed when we dropped
+ * the lock.
+ */
+ goto restart;
+ }
}
BUG_ON(node->nr_pages != 0);
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